The field of human stereotactic neurosurgery is over 40 years old. Stereotactic neurosurgery has usually involved the attachment of a frame to the patient's skull. Various imaging method are used to relate the position of the frame to the patient's anatomy. Thereafter, that data is used to guide the operative approach to a desired target which is seen on the image data. The target, which represents a physical point in the patient's head, may be pre-calculated in a variety of ways depending on the image data. Its known three-dimensional coordinates relative to the stereotactic frame can be further determined, and typically a mechanical arc system fastened to the patient-attached frame is used to stabilize and guide a probe to a target. This technique is widely known in neurosurgery and represents one of the fundamental techniques in that field.
More recently, attempts have been made to eliminate the need for a frame that is attached rigidly to the patient's skull. Thus, the field of "frameless stereotaxy" has become very active in the past year. Several investigators have made attempts at a frameless stereotactic mechanical arm as well as a frameless, non-mechanically coupled system to determine the position of a probe relative to the patient's anatomy without the need for a head frame. Most notable is the work by the present author, Dr. Barton Guthrie, who for many years has worked on an evolution of operating arms for the purpose. Other persons and organizations who have made similar operating arms following the lead of Dr. Guthrie are Dr. Maciunas, Dr. Watanabe, the ISG Company of Canada, and Dr. Reinhardt of Switzerland. In each of the latter four cases, the investigators have used an operating arm with six angular degrees of freedom. What is meant by this is an arm with articulating joints that gives rise to angular movements of arm linkages. One of the authors, Eric Cosman, has also investigated the use of optical coupling to determine a pointer's or probe's position, rather than mechanical coupling.
Dr. Guthrie has evolved an operating arm over the past several years in experimental and pre-clinical release investigations. In some embodiments, he has used five articulating joints, although six or more could also be used. Mathematically, five articulating joints are sufficient to place a pointer at a give position in space from a arbitrary angular direction. This statement is moderated only by the fact that real mechanical operating arms have certain joint and linkage limits which in turn limit the positional and angular approach range of the device. However, within this operating range of the device, five degrees of freedom are sufficient. Thus, one of the embodiments of the development of Guthrie has involved one less degree of freedom than all other investigators have incorporated in their systems and separates the author's invention from all other such devices that have been conceived to date. The use of more degrees of freedom increases the overall flexibility of the operating arm. It also can increase in some situations the operating range, both in position and in angular orientation of a probe. This would be a further positive aspect of using six or more degrees of freedom. However, the more degrees of freedom one uses, the more complex the apparatus becomes and thus the more difficult it is to maintain accuracy. With the proper configuring of five degrees of freedom, the present invention enables wide flexibility of approach and position while maintaining an intrinsic simplicity that all other such devices do not have. The stability and accuracy of the present invention is reflected in this simpler construction with fewer degrees of freedom. Specific arrangements of the five degrees of freedom of the present invention make it practical and functionally easy to use. Avoidance of so-called "gimble-lock" is achieved only by proper configuration of the joints and the links in an operating arm. This has also been achieved in the present invention with a unique geometric configuration. We have also developed unique six degree of freedom arms, and they will be described below as part of this information.
Other types of operating arms or pointers have been developed using ultrasonic and optical detection means to determine the pointers's position. These too may be considered "frameless" devices, although in all cases, they can be used with a stereotactic frame as well. What all other investigators have lacked in their devices is a simple way for the operator of the arm or pointer to convert it to a "3D mouse" so that the pointer itself can be used to change the functionality of the computer graphic system, that displays or reads out the pointer's position.
Thus one main objective of the present invention is to have the operating pointer arm, no matter what is operating principle may be, serve alternatively as a space pointer and at another instance a "3-D mouse" for interactive graphic control of the arm itself. This was achieved by switching the operation of the arm from its primary use as a pointer to its secondary use as a mouse for the screen graphics. The switching can be done in a variety of ways: footswitch, hand switch, third-party-operated switch, or by the position itself of the pointer in space. This greatly increases the convenience of the system and the facility of its use in a practical setting.
Yet a further objective of the present invention is to provide fiducial point means for calibrating the operating pointer or arm that involves a bite piece, or dental impression piece, that can be attached to the patient's dentition and which included localizer points, rods, or other structures to calibrate the position of the arm relative to the anatomy. In the operating setting, this will enable a skull-referenced fiducial marker system that can be accessed quickly and easily by the surgeon to recalibrate the position of his operating arm relative to the graphic anatomy which is displayed on the video monitor.
Yet another objective of the present invention is to provide novel joint and link configurations, both five and six degrees of freedom, to achieve superior operating flexibility of the arm in use.